By Ajay Kathuria, P.E., Senior Project Engineer
Published:  Meadowlands USA, Summer 2007

In the past five years, “vapor intrusion” has evolved into an increasingly familiar term to the owners, operators, and prospective buyers of buildings in close proximity to shallow soil and/or ground water contamination with gasoline, solvents, and other mixtures of toxic volatile chemicals (collectively referred to as VOCs).  Since the United States Environmental Protection Agency’s (USEPA) first published guidance on this issue in 2002, state regulatory agencies across the United States have shown growing interest in this health concern, and have issued their own versions of the vapor intrusion guidance documents and/or strict regulatory requirements to address vapors in the buildings.

Vapor intrusion refers to the migration of VOCs from the contaminated subsurface into the overlying buildings, where they may accumulate and persist at levels either high enough to pose an immediate or acute health risk, or a potential long term (chronic) incremental health risk to the building occupants.  The VOCs of concerns at a majority of the sites subject to vapor intrusion investigations include gasoline related compounds (e.g. benzene) and solvents such as trichloroethylene (TCE), used for degreasers, perchloroethylene (PCE), used in dry cleaning operations, and related breakdown compounds.

Indoor air sampling is being increasingly used at such buildings as part of the vapor intrusion investigations either driven by regulatory requirements, or conducted at a preliminary level in order to evaluate potential health risks.  However, the presence of often overlooked background and/or indoor sources of the VOCs of concern can make it extremely difficult to establish the true impacts of the “vapor intrusion”, and may lead to inappropriate and/or ineffective remedial measures at added costs.

A carefully designed vapor intrusion investigation or pathway assessment uses multiple lines of evidence in order to establish that the vapor intrusion is indeed occurring at a building, and at a level that poses health risks.  In general, a well delineated soil and/or ground water contamination plume, sub-slab soil sampling, indoor air testing and at least one (1) background (outdoor) air sample results provide the best line of evidence to evaluate vapor intrusion.  However, in addition to conducting a building survey to determine any preferential pathways (e.g. floor cracks, sumps, etc.) that allow for easy vapor migration, a comprehensive background investigation into the potential indoor sources of harmful vapors can become a critical factor in a vapor intrusion investigation and for facilitating appropriate remedial design decision making.

As per the vapor intrusion guidance issued by the New Jersey Department of Environmental Protection (NJDEP) as well as similar guidance in other states, there are several important and common background sources that must be investigated as part of a vapor intrusion investigation.  These include household activities, consumer products, building materials and furnishings, and ambient or outdoor air pollution.  For sites with gasoline or petroleum spills related concerns, contributing background sources may include indoor storage or use of gas-powered equipment and improperly stored gasoline containers.  Indoor smoking can also contribute to the build up of benzene levels in the indoor air in addition to more than 50 carcinogenic compounds.  Regular use of freshly dry cleaned clothes or the presence of in-house dry clean pick-up/ drop-off service can contribute to PCE levels in the indoor air above the risk-based potential long-term health risk levels.  The use of certain consumer products such as spot rug cleaners can also contribute to the presence of PCE and TCE in the indoor air.  Other common consumer products to be considered include mothballs or urinal cakes (1,4-dichlorobenzene), nail polish remover (acetone), floor polish (xylenes), drain cleaners (1,1,1-trichloroethane) and scented candles (TCE).  Building materials such as particle board, fabric, furniture can contribute to formaldehyde levels in the indoor air.  In urban settings, outdoor air pollution (diesel and industrial fumes) can be an important factor in vapor intrusion investigations.

In general, potential background sources must always be investigated and removed from the building prior to any indoor and/or sub-slab sampling event.  In cases where VOCs of concern are also being used inside the building, the indoor air levels can be compared with the standards set by the Occupational Safety and Health Administration, which are generally much higher than the risk levels published in vapor guidance documents.

In many cases, a careful background investigation can provide critical or missing information in an otherwise well designed vapor intrusion investigation, and for the selection of appropriate remedial or mitigative measures.  In many cases, the results of sub-slab and outdoor air sampling suggest the absence of vapor intrusion but an indoor source may be identified during background investigation and it can be readily addressed.  In cases where the sub-slab to indoor air sample levels strongly suggest vapor intrusion, but contributing background sources have not been investigated, remedial measures such as the installation of a sub-slab ventilating system at a substantial cost may not be effective until such indoor sources are identified and addressed.

Background sources cannot be overlooked in determining the cause of indoor air quality concerns.  They need to be considered in a vapor intrusion investigation and in designing the appropriate remedial measures, because in many cases they are the sole causes of the exposure.

Ajay Kathuria is a Senior Project Engineer with Environmental Waste Management Associates, LLC in Parsippany, New Jersey.  www.ewma.com